Field of the Invention
This invention relates to polymeric materials, more particularly it relates to thermosetting polymers, and more particularly it relates to thermosetting shape memory polymer matrices that comprise particles of thermoplastic polymer dispersed therein, allowing the material to have repeatable, self-healing properties.
Description of Related Art
A polymer is a large molecule (macromolecule) composed of repeating structural units. These sub-units are typically connected by covalent chemical bonds. The term polymer encompasses a large class of compounds comprising both natural and synthetic materials with a wide variety of properties. Because of the extraordinary range of properties of polymeric materials, they play essential and ubiquitous roles in everyday life. These roles range from familiar synthetic plastics and elastomers to natural biopolymers such as nucleic acids and proteins that are essential for life.
A plastic material is any of a wide range of synthetic or semi-synthetic organic solids that are moldable. Plastics are typically organic polymers of high molecular mass, but they often contain other substances.
There are two types of plastics: thermoplastic polymers and thermosetting polymers. Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again. Examples include polyethylene, polypropylene, polystyrene, polyvinyl chloride, and polytetrafluoroethylene (PTFE). Common thermoplastics range from 20,000 to 500,000 amu.
In contrast, thermosets are assumed to have an effectively infinite molecular weight. These chains are made up of many repeating molecular units, known as repeat units, derived from monomers; each polymer chain will have several thousand repeating units. Thermosets can melt and take shape once; after they have solidified, they stay solid. In the thermosetting process, a chemical reaction occurs that is irreversible. According to an IUPAC-recommended definition, a thermosetting polymer is a prepolymer in a soft solid or viscous state that changes irreversibly into an infusible, insoluble polymer network by curing. The cure may be done through heat (generally above 200° C. (392° F.)), through a chemical reaction (two-part epoxy, for example), or irradiation such as electron beam processing. A cured thermosetting polymer is often called a thermoset.
Thermoset materials are usually liquid or malleable prior to curing and designed to be molded into their final form, or used as adhesives. Others are solids like that of the molding compound used in semiconductors and integrated circuits (IC). In contrast to thermoplastic polymers, once hardened a thermoset resin cannot be reheated and melted back to a liquid form.
The curing process transforms the thermosetting resin into a plastic or rubber by a cross-linking process. Energy and/or catalysts are added that cause the molecular chains to react at chemically active sites (unsaturated or epoxy sites, for example), linking into a rigid, 3-D structure. The cross-linking process forms a molecule with a larger molecular weight, resulting in a material with a heightened melting point. During the curing reaction, the molecular weight has increased to a point so that the melting point is higher than the surrounding ambient temperature, the material forms into a solid material.
However, uncontrolled heating of the material results in reaching the decomposition temperature before the melting point is obtained. Thermosets never melt. A thermoset material cannot be melted and re-shaped after it is cured. A consequence of this is that thermosets generally cannot be recycled, except as filler material.
Thermoset materials are generally stronger than thermoplastic materials due to their three dimensional network of bonds (cross-linking). Thermosets are also better suited to high-temperature applications (up to their decomposition temperature). However, they are more brittle. Because of their brittleness, thermoset is vulnerable to high strain rate loading such as impact damage. Since a lot of lightweight structures use fiber reinforced thermoset composites, impact damage, if not healed properly and timely, may lead to catastrophic structural failure.
Thermoplastic, also known as a thermosoftening plastic, is a polymer that turns to a viscous liquid when heated and freezes to a rigid state when cooled sufficiently. Most thermoplastics are high-molecular-weight polymers whose chains associate through weak van der Waals forces (polyethylene); stronger dipole-dipole interactions and hydrogen bonding (nylon); or even stacking of aromatic rings (polystyrene). As noted herein, thermoplastic polymers differ from thermosetting polymers (e.g. phenolics, epoxies) in that they can be remelted and remolded.
Many examples of thermoplastic polymers are known in the art, including Acrylonitrile butadiene styrene, Acrylic (e.g., PMMA), Celluloid, Cellulose acetate, Cyclic Olefin Copolymer, Ethylene-Vinyl Acetate, Ethylene vinyl alcohol, Fluoroplastics (e.g., PTFE, FEP, PFA, CTFE, ECTFE, ETFE), Ionomers, Polyoxymethylene (POM or Acetal), Polyacrylates, Polyacrylonitrile, Polyamide (e.g., Nylon), Polyamide-imide, Polyaryletherketone, Polybutadiene, Polybutylene, Polybutylene terephthalate, Polycaprolactone, Polychlorotrifluoroethylene, Polyethylene terephthalate, Polycyclohexylene dimethylene terephthalate, Polycarbonate, Polyhydroxyalkanoates, Polyketones, Polyesters, Polyethylenes, Polyetheretherketone, Polyetherketoneketone, Polyetherimide, Polyethersulfone, Chlorinated Polyethylene, Polyimide, Polylactic acid (PLA), Polymethylpentene, Polyphenylene oxide, Polyphenylene sulfide, Polyphthalamide, Polypropylene, Polystyrene, Polysulfone, Polytrimethylene terephthalate, Polyurethane, Polyvinyl acetate, Polyvinyl chloride, Polyvinylidene chloride, and Styrene-acrylonitrile copolymer.
Thermoplastics can go through melting/freezing cycles repeatedly and the fact that they can be reshaped upon reheating gives them their name. However, this very characteristic of reshapability also limits the applicability of thermoplastics for many industrial applications, because a thermoplastic material will begin to change shape upon being heated above its Tg and Tm.
Initiation of cracks and other types of damage on a microscopic level has been shown to change thermal, electrical, and acoustical properties, and eventually lead to wholesale failure of the material. From a macromolecular perspective, stress induced damage at the molecular level leads to larger scale damage called microcracks. A microcrack is formed where neighboring polymer chains have been damaged in close proximity, ultimately leading to the weakening of the fiber as a whole. In view of the diverse use of polymers in industry, it is self-evident that failure of safety-critical polymer components such as brittle thermoset polymers is a serious problem; failure of these materials can lead to serious even catastrophic accidents.
For thermoset polymers that have developed cracks, unfortunately there are only two fundamental choices, attempt to repair the crack or entirely remove and replace the component that contains the damaged material. Usually, cracks are mended by hand, which is difficult because cracks are often hard to detect. A polymeric material that can intrinsically correct damage caused by normal usage could lower production costs through longer part lifetime, reduction of inefficiency over time caused by degradation of the part, as well as prevent costs incurred by material failure.
Since its introduction in the 1980s in an attempt to heal damage, restore mechanical properties and extend the service life of polymers, the concept of crack healing in polymeric materials has been widely investigated [201-215]. In thermoplastic polymers, the most widely studied and reported mechanism for self-healing is the molecular inter diffusion mechanism. It has been reported [201] that when two pieces of the same polymer are brought into contact at a temperature above its glass transition temperature (Tg), the interface gradually disappears and the mechanical strength at the polymer-polymer interface increases as the crack heals due to molecular diffusion across the interface. To better explain the process of crack healing by this mechanism, various models have been proposed [202-204, 216]. In particular, Wool and O'Connor [216] suggested a five stage model to explain the crack healing process in terms of surface rearrangement, surface approaching, wetting, diffusion and randomization. Kim and Wool [206] also presented a microscopic theory for the diffusion and randomization stages. In another study [207], it was observed that the development of mechanical strength during the crack healing process of polymers is related to interdiffusion of the molecular chains and subsequent formation of molecular entanglements. Other reported healing mechanisms in thermoplastic polymers include photoinduced healing, recombination of chain ends, self-healing via reversible bond formation, and via nanoparticles [217].
In thermoset polymers, self-healing mechanisms acting through the incorporation of external healing agents such as liquid healing agent (monomer) encased in hollow fibers [218, 219], micro-capsules [220, 221], and solid healing agent (thermoplastic particles) dispersed in the thermoset matrix [217, 222], have been proposed and tested. However, the different physical and behavioral characteristics of thermoset SMPs relative to standard thermosets make the applicability or suitability of a component on one type of the thermosets of uncertain relevance to others. For example, with regular thermosets suitability of an additional component depends on the chemical compatibility, viscosity of the molten thermoplastic, and the concentration gradient, whereas for thermoset SMPs, suitability of an additional component also depends on diffusion under the recovery force. Some polymers by themselves possess the self-healing capability such as thermally reversible crosslinked polymers [223] and ionomers [224]. Although these systems are very successful in healing micro-length scale damage, they face tremendous challenge when they are used to repair large, macroscopic, structural-length scale damage, which are visible to the naked eye [225-227].
Self-healing of structural damage has been a tremendous interest in the scientific community recently. A true challenge is to mimic biological systems and repair the internal damage autonomously, repeatedly, efficiently, and at molecular-length scale. The state-of-the-art self-repair of thermosetting polymers and their composites includes: (1) use of hollow fibers/microcapsules to release polymeric resin when ruptured, and heal the crack through in-situ polymerization triggered by a catalyst contained in the polymer matrix; (2) use of thermoplastic particles to flow into the crack when heated up and glue the crack when cooled down; and (3) use of thermo-reversible covalent bonds via a retro-Diels-Alder (DA) reaction. Despite the significant advancements made using a bio-mimetic approach, there is still a long way to go before even the simplest biological healing mechanism can be replicated with synthetic materials. One major difference between biological and prior synthetic healing mechanisms is that biological systems involve multiple-step healing solutions. For example, mammalian healing processes rely on fast forming patches to seal and protect damaged skin before the slow regeneration of the final repair tissue.
Several self-healing schemes have been reported in the literature primarily for healing microcracks, including incorporation of external healing agents such as liquid healing agent in microcapsules, hollow fibers, and microvascular networks, and solid healing agent such as embedded thermoplastic particles. Some polymers by themselves possess self-healing capabilities, including ionomers, which consist of over 15% of ionic groups, and a highly cross-linked polymer, which is synthesized via the Diels-Alder (DA) cycloaddition of furan and maleimide moieties, and the thermal reversibility of the chemical bonds is accomplished via the retro-DA reaction. A combination of microcapsule and shape memory alloy (SMA) wire has also been studied. Because damage is usually in structural-length scale, the challenge is how to heal macrocracks. However, the existing systems are unable to very effectively heal macroscopic damage. For instance, in order to heal macrocracks, a large amount of healing agent is needed. However, incorporation of a large amount of healing agent will significantly alter the physical/mechanical properties of the host structure. Also, large capsules/thick hollow fibers themselves may become potential defects when the encased healing agent is released. A major challenge is how to heal structural-length scale damage such as impact damage autonomously, repeatedly, efficiently, timely, and at the molecular-length scale. Recently, shape memory polymer (SMP) has emerged as a new type of smart material. Various types of applications have been studied, particularly in lightweight structure applications. Tey et. al [426] studied the shape memory functionality of a polyurethane (PU) based SMP foam by performing the conventional thermomechanical programming cycle and recommended those PU based foams be used in foldable space vehicles and quick molding devices. Huang et. al [427] studied the influence of cold hibernation on the shape memory properties of PU based SMP foams. They concluded that the cold hibernation process did not affect the shape memory properties in spite of keeping them in a compacted state for a prolonged period.
A number of types of shape memory polymers are known in the art. Shape memory can be engineered into a number of polymers, including block copolymers such as those containing polyurethanes; polyurethanes with ionic or mesogenic components; polyurethanes crosslinked with glycerin or trimethylol propane; block copolymer of polyethylene terephthalate (PET) and polyethyleneoxide (PEO); block copolymers containing polystyrene and poly(1,4-butadiene); ABA triblock copolymer made from poly(2-methyl-2-oxazoline) and polytetrahydrofuran; PEO-PET block copolymers crosslinked with maleic anhydride, glycerin or dimethyl 5-isopthalates; AA/MAA copolymer crosslinked with N,N′-methylene-bis-acrylamide; MAA/N-vinyl-2-pyrrolidone copolymer crosslinked with ethyleneglycol dimethacrylate; PMMA/N-vinyl-2-pyrrolidone crosslinked with ethyleneglycol dimethacrylate; styrene acrylate; cyanate ester; and epoxy polymer. The “memory,” or recovery, quality comes from the stored mechanical energy attained during the reconfiguration and cooling of the material. Above its transition temperature, an SMP goes from a rigid, plastic state to a flexible, elastic state. When cooled, it becomes rigid again and can be constrained in its new shape configuration. Shape memory characteristics can be engineered into different types of polymers.
The shape memory effect involves two components: Cross-linkers, which determine the “permanent” shape, and “switching segments,” which maintain a temporary shape. Above the glass transition temperature a shape memory polymer will be in its “permanent” or “memory” shape, in the absence of a load. It can be deformed at the elevated temperature into another shape, and then cooled to lock in the deformed or “temporary” shape. Upon re-heating above the glass transition temperature, the polymer returns to its memory shape. This cycle can be repeated many times without degradation.
One problem with existing self-healing systems is the presence of voids after the healing process. For example, a polymeric material will contain microcapsules of monomer throughout, and similarly an initiator/catalyst would be uniformly present throughout the material. When a crack occurs, the monomer-bearing capsules at the site of the crack would rupture, disgorge monomer and polymerization would result because of the presence of the initiator. Prior to polymerization, the capillary forces at the crack face would encourage even flow of monomer, resulting in an evenly-healed crack. However, after the crack has healed the material now has voids where the monomer capsules used to be. These voids can have an adverse effect upon the material's mechanical properties. Moreover, this self-healing process is available for only one time in the area of the healed crack.
In a previous study, it was shown that the stress-controlled programming and partially confined shape recovery of a SMP based syntactic foam was able to close impact damage repeatedly, efficiently, and almost autonomously (the only human intervention is by heating) [7]. It was found that the key to using a shape memory effect for self-closing of cracks depended on both the reduction of structure volume during programming, and the external confinement of the structure during shape recovery. This is because once damage is created in a programmed structure that has a reduced volume, the structure tends to recover its original, larger volume during the heating or shape recovery process, due to its shape memory functionality. If the expansion in volume was resisted by external confinement, the material will be pushed towards internal open spaces such as cracks, achieving the self-closing purposes. Therefore, compression programming was required. Repeatability in self-closing (up to 7 cycles [7]) comes from the fact that each round of confined shape recovery served dual purposes: one for self-closing internal cracks, and the other for completing a new round of compression programming.
This combination of closing and reprogramming is achieved because the confined shape recovery came about by heating the foam above the Tg, applying a certain compressive stress to the foam due to confinement, and cooling down below Tg while maintaining the prestrain, which is typical for strain-controlled programming. In other words, strain-controlled programming was coupled with confined shape recovery. Therefore, although it may seem as if only one programming was conducted at the very beginning of the repeated impact/closing cycles [7], each shape recovery actually had one prior programming to supply the energy. The subsequent programmings were automatically performed by being coupled with each confined shape recovery. Therefore, one “nominal” programming led to several cycles of shape recovery [7]. Of note, the method of [7] although referred to as “healing” was only a closing of the polymer defect; no molecular scale healing was achieved. The approach of [7] had disadvantages, it lacked the ability to regain a substantial amount of the original integrity as the two sides of a crack were brought back into contact with each other, but were not reconnected in a “healing” manner one to the other.
Fiber reinforced polymer composite sandwich structures have been used in various engineering structures. Damage due to transverse impact loads has been a critical problem for composite sandwich structures. In a composite sandwich structure, the core is primarily responsible for dissipating impact energy in addition to providing transverse shear resistance. Various types of core materials have been studied such as foam core (polymeric foam, metallic foam, ceramic foam, balsa wood, syntactic foam, etc.) [401, 402], web core (truss, honeycomb, etc.) [403], 3-D integrated core [404, 405], foam filled web core [405, 406], and laminated composite reinforced core [407]. However, these core materials are limited in one way or another. For example, brittle syntactic foam cores absorb impact energy primarily through macro-length scale damage, significantly sacrificing residual strength [408-411]; web cores lack bonding with the skin and also have impact windows (small regions in the core that are not filled with the foam, resulting in complete perforation of the sandwich under impact) [405, 406]; 3-D integrated core suffers from pile buckling [405], etc.
Li and Muthyala 2008 [412] and Li and Chakka [413] disclosed a sandwich structure with an integrated grid stiffened syntactic foam core. It is found that this sandwich develops synergy between the grid skeleton and the filled foam, and between the core and the skin, leading to a much higher post-impact residual in-plane compressive strength than the traditional laminated composite with the same amount of raw materials. However, the residual strength is still very low as compared to the intact sandwich. It is desired to maintain the post-impact structural strength through damage self-healing so that the impact damaged sandwich can be continuously used in service.
Thus, prior to the disclosure of the present invention, there has been a need in the art for a SMP self-closing mechanism that not only closes defects, but allows for molecular scale healing of the defect; in particular there is a need for such healing that can be performed on a repeated basis at or near the site of a prior wound.